1. Field of the Invention
This invention relates to the hydrodesulfurization of high boiling hydrocarbon feedstocks. More particularly, it relates to a hydrodesulfurization process employing a finely divided catalyst which remains in suspension throughout the process.
2. Description of the Prior Art
Prior art hydrodesulfurization processes have been traditionally carried out by passing the hydrocarbon feedstock downflow through fixed catalyst beds or upflow through an ebullating catalyst bed. The ebullating bed system is described in Layng et al, U.S. Pat. No. 3,553,105 and comprises introducing the liquid feedstock and hydrogen into the bottom of a contact zone containing either an extruded particulate catalyst ranging in size from 1/32 to 1/16 inch diameter or a micro-spheroidal catalyst ranging from about 20 to 325 U.S. mesh (841 to 44 microns). The feedstock is passed upwardly through a contact zone at a sufficient space velocity to expand the catalyst bed by at least 10%. The vapor and liquid products do not contain the catalyst and are removed from the top of the contact zone for phase separation and other downstream treatment. The catalyst in such a process must be periodically regenerated and recycled to the contact zone. This procedure involves a loss in production or on-stream time due to shutdown for catalyst regeneration or for replacement of the bed with fresh catalyst. In addition, hydrogen consumption in the prior art processes is high because of undesired hydrocracking and hydrogenation reactions due to the high resistance of hydrogen diffusion into the pores of the relatively large catalyst particles. Increased hydrogen diffusion rates which accompany the much smaller particles of the process of the present invention will reduce the undesired hydrogen consuming reactions.
A recent development in hydrodesulfurization has been the process described in Jacobsen, U.S. Pat. No. 3,841,996. In this process, a hydroconversion catalyst in particulate form having a typical particle size in the range from 0.02 to 0.5 mm (20 to 500 microns) is dispersed in the heavy petroleum feedstock and circulated within a reaction loop at a weight hourly space velocity (WHSV) of from 0.5 to 50 kg. of oil per kg. of catalyst per hour and at an elevated temperature and pressure to effect desulfurization. The feedstock must be circulated within the loop at a sufficient velocity to maintain the relatively large catalyst particles in the dispersion. The effluent from the reaction loop which still contains a portion of the catalyst is separated into a gas phase, a liquid product phase and a solid phase which contains that portion of the catalyst in the effluent in the form of a thick slurry in oil or a paste. This catalyst slurry or paste is recycled to the hydrodesulfurization process. Periodically the catalyst must be subjected to regeneration. Part of the spent catalyst is discarded and replaced with fresh catalyst. In view of the foregoing, this process has some of the same disadvantages as in the case with the traditional processes mentioned above to achieve the necessary process economics.